Peter Sprafke

Damage-mechanic material characterization and structural finite element analysis of polymer encapsulation materials

Microsystems are exposed to thermo-mechanical loads causing strains and stresses in the used materials because of the mismatch of thermal expansion. To protect microsystems against thermo-mechanical loads and environmental impacts, polymer encapsulation is widely used. Thus the polymer encapsulation substantially influences the reliability of the component. In order to forecast the reliability of such a component, the damage mechanical behavior of the polymer encapsulation material has to be investigated. At a microscale thermally induced stresses lead to initiation of microcracks and microvoids. Damage mechanics is applied to describe the progressive deterioration of the material. Within damage mechanics the state of the damaged material is characterized by a damage measure. Basic concepts of damage mechanics are outlined in this paper. As the used polymer material shows viscoelastic material behavior, we characterize the viscoelastic material properties by a relaxation experiment. Simulation of the established material model is compared with experimental data. As simulation and experiment show good agreement, this material model can be used for further damage analysis. The accumulation of damage in the polymer material is measured by a uniaxial tension test with repeated unloading. The experiment shows monotonically increasing damage of the material, which reflects the irreversibility of damage. Comparison with a simulation of undamaged viscoelastic material behavior shows, that the measured damage accumulation is a significant effect.